This invention relates to hydraulic actuator assemblies and more particularly to a preassembled and prefilled hydraulic actuator system for use in actuating the clutch of a motor vehicle.
Hydraulic actuators are known for operating motor vehicle clutches and other mechanisms at a remote location by way of a master cylinder and a slave cylinder combination. The master cylinder is operated, for example, by the clutch pedal, and the slave cylinder is connected to the clutch via a throwout bearing. The master and slave cylinders are interconnected by a high pressure line or conduit. Depressing the clutch pedal displaces the piston in the master cylinder to close a passage or port between the master cylinder fluid chamber and a fluid reservoir and to pressurize the connection to the slave cylinder. Displacement of the piston in the slave cylinder disengages the clutch.
It is also known to preassemble and prefill apparatus of this type before shipment of the assembly to a motor vehicle manufacturer for installation on the motor vehicle. The motor vehicle manufacturer need only to fix the master and slave cylinders in place and make simple mechanical connections to the clutch pedal and clutch bearing. See U.S. Pat. No. 4,599,860 entitled "Liquid Pressure Apparatus" issued July 15, 1986 to David Parsons and assigned to Automotive Products, plc of Warwickshire, England.
Prefilled and preassembled hydraulic actuator apparatus of the type shown in the aforementioned Parsons' patent have achieved wide acceptance, particularly in the automotive industry, because of the ease of installation in the motor vehicle, because of their extremely reliable operation over long periods of time, and because of their ability to automatically compensate for wear.
Various master cylinder designs have been employed as the master cylinder unit of these prefilled hydraulic actuator assemblies. One such design, known as a valve style master cylinder, employs a one-way valve on the master cylinder which operates in the depressurized system to provide replenishing fluid to the system when needed and operates in response to pressurization of the system to block communication between the master cylinder and the reservoir so as to optimize the operation of the slave cylinder. These valve style master cylinders, one of which is shown for example in U.S. Pat. No. 4,532,768, have the advantage of providing the most compact and least expensive master cylinder construction of all of the known prior art master cylinder constructions while retaining effective replenishing of the system when required and effective isolation of the reservoir from the system during pressurization of the system.
However, these valve style master cylinders sometimes present a problem because of their inability to allow the return of fluid to the reservoir when the system is pressurized. For instance, when the clutch of a motor vehicle is removed for repair, the slave cylinder extends to its fully extended position under the influence of the slave cylinder spring and the valve in the master cylinder opens to allow entry of replenishing fluid into the system in compensation for the extending movement of the slave piston. However, when it is desired to again retract the slave cylinder during reinstallation of the clutch in the motor vehicle, the valve on the master cylinder immediately senses the pressurization of the system resulting from the attempted retraction of the slave cylinder and precludes movement of fluid from the system into the reservoir with the result that the slave cylinder can be retracted only with great difficulty or not at all.
As a further instance, when the operator of the vehicle purposely or inadvertently slips his foot sideways off of the clutch pedal and allows the clutch to pop, the spring of the master cylinder moves the piston of the master cylinder somewhat more rapidly than the slave cylinder piston is moving under the influence of the clutch spring with the result that the system becomes depressurized and the valve on the master cylinder opens to allow replenishing fluid to enter the system. After several such instances of additional fluid being pumped into the system as a result of popping of the clutch, the clutch becomes difficult to operate and/or the clutch operates continually in a partially engaged or slipping condition.